The present invention relates to methods of producing novel transfer sheets used in pressure-sensitive carbonless copy systems, the resultant copy systems, and methods of producing self-contained transfer sheets for carbonless copy systems. Additionally, this invention relates to methods of improving the stability and performance of the leuco chromogenic compounds used in the preparation of carbonless copy systems.
Carbonless copying systems employing chromogenic materials have been used commercially in various applications for several years. One of the most common uses of such carbonless systems has been in pressure sensitive copy systems wherein a colorless chromogenic material dissolved in an oily solvent is encapsulated and coated onto the back of a transfer substrate, usually referred to as the transfer sheet or CB part of a carbonless system. With the application of localized pressure such as from a stylus or a typewriter, pen, or other implement, the microcapsules rupture and the chromogen solution is transferred to the front of an underlying sheet having an absorbent coating of a Lewis acid, or other color-developing material which reacts with the chromogen released from the ruptured microcapsules to produce visible colored images; the underlying or receiving sheet is usually referred to as the receptor sheet or CF part of a carbonless system. Multiple-ply business forms can be produced by properly assembling a CB part and a CF part with a number of CFB (coated front and back) parts interposed between the CB and the CF parts.
In addition to the conventional carbonless copy systems described above, there are also commercially available self-contained carbonless copying systems in which the encapsulated chromogen and the Lewis acid materials are incorporated on the same substrate surface, usually the front, and upon application of pressure to the surface, the capsules are ruptured releasing the chromogen solution which is immediately absorbed by the surrounding environment of the Lewis acid material, and produces the visible colored image.
Heretofore, self-contained carbonless systems have been produced by a variety of methods. In the "multiple-coating" method, the chromogen containing microcapsules are first coated unto the substrate, followed by the coating of a film-forming substance such as polyvinyl alcohol, gelatin and the like, on top of the microcapsule coating, and followed by the coating of the Lewis acid material on top of the film-forming substance. The "multiple-coating" method is tedious, time consuming, and rather costly. Additionally, the final product, usually a paper substrate containing all three coatings on the same surface, is not not very aesthetically attractive. The repeated wetting and drying of the paper during the three coating applications without the ability to calendar the paper between coatings because of the presence of the rupturable microcapsules, results in an uneven, or "rough" final surface which besides being aesthetically unattractive also produces discontinuous and unclear images when such a system is used to make multiple copies.
Another method of manufacturing self-contained carbonless copy systems involves the individual encapsulation of the chromogen solution and the Lewis acid material, the subsequent admixing of the two microcapsular dispersions, and application of the mixture as a coating on the same paper surface. In such methods, extreme care must be exercised in producing microcapsules of high structural integrity and in handling the final admixed dispersions to avoid premature interaction of the two main components (chromogen and Lewis acid) which will result in the generation of colored images in unwanted sites.
An improved method of producing self-contained carbonless copy systems is described in U.S. Pat. No. 4,586,060 in which only the oily solution of the chromogen is encapsulated using microcapsules with higher wall thickness and substantially lower permeability, and which capsules are dispersed in an unencapsulated solution of the Lewis acid and coated onto the same surface of the substrate.